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<body>
<h1 id="-vuex">状态管理与Vuex</h1>
<p>非父子组件（跨级组件和兄弟组件）通信时，使用了<code>bus</code>（中央事件总线）的一个方法，用来触发和接收事件，进一步起到通信的作用。</p>
<p>一个组件可以分为数据（<code>model</code>）和视图（<code>view</code>），数据更新时，视图也会自动更新。在视图中又可以绑定一个事件，它们触发<code>methods</code>里指定的方法，从而可以改变数据、更新视图，这是一个组件基本的运行模式。</p>
<pre><code>const store = new Vuex.Store({});
</code></pre><p>仓库<code>store</code>包含了应用的数据（状态）和操作过程。Vuex里的数据都是响应式的，任何组件使用同一<code>store</code>的数据时，只要<code>store</code>的数据变化，对应的组件也会立即更新。</p>
<p>数据保存在Vuex选项的<code>state</code>字段内。</p>
<pre><code>const store = new Vuex.Store({
    state: {
        count: 0
    }
});
</code></pre><p>在任何组件内，可以直接通过<code>$store.state.count</code>读取。</p>
<pre><code>&lt;template&gt;
    &lt;div&gt;
        &lt;h1&gt;首页&lt;/h1&gt;
        {{$store.state.count}}
    &lt;/div&gt;
&lt;/template&gt;
</code></pre><p>直接卸载<code>template</code>里显得有点乱，可以用一个计算属性来显示：</p>
<pre><code>&lt;div&gt;
    &lt;h1&gt;首页&lt;/h1&gt;
    {{count}}
&lt;/div&gt;

export default {
    computed: {
        count() {
            return $store.state.count;
        }
    }
}
</code></pre><p>在组件内来自<code>store</code>的数据只能读取，不能手动修改，修改<code>store</code>中数据的唯一途径是显式地提交<code>mutations</code>。</p>
<p><code>mutations</code>是Vuex的第二个选项，用来直接修改<code>state</code>里的数据。</p>
<p>在组件内，通过<code>this.$store.commit</code>方法来执行<code>mutations</code>。</p>
<p><code>mutations</code>还可以接受第二个参数，可以是数字、字符串或对象等类型。</p>
<p><strong>ES6语法</strong></p>
<p>函数的参数可以设定默认值，当没有传入该参数时，使用设置的值。</p>
<p>increment(state,n=1)等同于：<br>    increment(state,n){<br>        n=n||1;<br>    }</p>
<p>提交<code>mutations</code>的另一种方式是直接使用包含<code>type</code>属性的对象。</p>
<blockquote>
<p><code>mutations</code>里尽量不要异步操作数据，否则组件在<code>commit</code>后数据不能立即改变，而且不知道什么时候会改变。</p>
</blockquote>
<h2 id="-">高级用法</h2>
<p>Vuex还有其他3个选项可以使用：<code>getter</code>、<code>actions</code>、<code>modules</code>。</p>
<p><code>getter</code>能将<code>computed</code>的方法提取出来，也可以依赖其他的<code>getter</code>，把<code>getter</code>作为第二个参数。</p>
<p><code>action</code>与<code>mutation</code>很像，不同的是<code>action</code>里面提交的是<code>mutation</code>，并且可以一步操作业务逻辑。</p>
<p><code>action</code>在组件内通过<code>$store.dispatch</code>触发。</p>
<p><code>modules</code>用来将<code>store</code>分割到不同模块，当项目足够大时，<code>store</code>里的<code>state</code>、<code>getters</code>、<code>mutations</code>、<code>actions</code>会非常多，使用<code>modules</code>可以把它们写到不同的文件中。</p>
<p><code>module</code>的<code>mutation</code>和<code>getter</code>接收的第一个参数<code>state</code>是当前模块的状态。在<code>actions</code>和<code>getters</code>中还可以接收一个参数<code>rootState</code>，来访问根节点的状态。</p>
<h1 id="-vue-bus-">实战：中央事件总线插件<code>vue-bus</code></h1>
<p>中央事件总线<code>bus</code>作为一个简单的组件传递事件，用于解决跨级和兄弟组件通信的问题。</p>
<p><code>vue-bus</code>插件给Vue添加一个属性<code>$bus</code>，并代理<code>$emit</code>、<code>$on</code>、<code>$off</code>三个方法。</p>
<p><strong>ES6语法</strong></p>
<blockquote>
<p><code>emit(event,..args)</code>中的<code>...</code>是函数参数的解构。因为不知道组件会传递多少个参数进来，使用<code>...args</code>可以把从当前参数到最后的参数都获取到。</p>
</blockquote>
<p>使用<code>vue-bus</code>有两点需要注意：</p>
<ol>
<li>第一是<code>$bus.on</code>应该在<code>created</code>钩子内使用，如果在<code>mounted</code>使用，它可能接收不到其他组件来自<code>created</code>钩子内发出的事件；</li><li>第二点是使用了<code>$bus.on</code>在<code>beforeDestroy</code>钩子里应该再使用<code>$bus.off</code>解除，因为组件销毁后，就没有必要把监听的句柄存储在<code>vue-bus</code>中。</li></ol>

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